Change mongodb URL to make use of service name

.gitlab-ci.yml

@@ -26,7 +26,7 @@ test_server:
     - cd app/server
     - npm install
     - PORT=3002 \
-      MONGODB_URL=mongodb://localhost:27017/test \
+      MONGODB_URL=mongodb://mongodb:27017/test \
       JWT_SECRET=$JWT_SECRET \
       npm run test